Re: [PATCH kvm-next V11 6/7] KVM: guest_memfd: Enforce NUMA mempolicy using shared policy

On Wed, Aug 27, 2025, Shivank Garg wrote:

@@ -26,6 +28,9 @@ static inline struct kvm_gmem_inode_info *KVM_GMEM_I(struct inode *inode) return container_of(inode, struct kvm_gmem_inode_info, vfs_inode); } +static struct mempolicy *kvm_gmem_get_pgoff_policy(struct kvm_gmem_inode_info *info,

• 				   pgoff_t index);
  

/**

• folio_file_pfn - like folio_file_page, but return a pfn.
• @folio: The folio which contains this index.

@@ -112,7 +117,25 @@ static int kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slot, static struct folio *kvm_gmem_get_folio(struct inode *inode, pgoff_t index) { /* TODO: Support huge pages. */

• return filemap_grab_folio(inode->i_mapping, index);

• struct mempolicy *policy;
• struct folio *folio;
• /*

• * Fast-path: See if folio is already present in mapping to avoid
  

• * policy_lookup.
  

• */
  

• folio = __filemap_get_folio(inode->i_mapping, index,

• 		    FGP_LOCK | FGP_ACCESSED, 0);
  

• if (!IS_ERR(folio))

• return folio;
  

• policy = kvm_gmem_get_pgoff_policy(KVM_GMEM_I(inode), index);
• folio = __filemap_get_folio_mpol(inode->i_mapping, index,

• 			 FGP_LOCK | FGP_ACCESSED | FGP_CREAT,
  

• 			 mapping_gfp_mask(inode->i_mapping), policy);
  

• mpol_cond_put(policy);
• return folio;

} static void kvm_gmem_invalidate_begin(struct kvm_gmem *gmem, pgoff_t start, @@ -372,8 +395,45 @@ static vm_fault_t kvm_gmem_fault_user_mapping(struct vm_fault *vmf) return ret; } +#ifdef CONFIG_NUMA +static int kvm_gmem_set_policy(struct vm_area_struct *vma, struct mempolicy *mpol) +{

• struct inode *inode = file_inode(vma->vm_file);
• return mpol_set_shared_policy(&KVM_GMEM_I(inode)->policy, vma, mpol);

+}

+static struct mempolicy *kvm_gmem_get_policy(struct vm_area_struct *vma,

• 			     unsigned long addr, pgoff_t *pgoff)
  

+{

• struct inode *inode = file_inode(vma->vm_file);
• *pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT);
• return mpol_shared_policy_lookup(&KVM_GMEM_I(inode)->policy, *pgoff);

+}

+static struct mempolicy *kvm_gmem_get_pgoff_policy(struct kvm_gmem_inode_info *info,

• 				   pgoff_t index)
  

I keep reading this is "page offset policy", as opposed to "policy given a page offset". Another oddity that is confusing is that this helper explicitly does get_task_policy(current), while kvm_gmem_get_policy() lets the caller do that. The end result is the same, but I think it would be helpful for gmem to be internally consistent.

If we have kvm_gmem_get_policy() use this helper, then we can kill two birds with one stone:

static struct mempolicy *__kvm_gmem_get_policy(struct gmem_inode *gi, pgoff_t index) { struct mempolicy *mpol;

mpol = mpol_shared_policy_lookup(&gi->policy, index); return mpol ? mpol : get_task_policy(current); }

static struct mempolicy *kvm_gmem_get_policy(struct vm_area_struct *vma, unsigned long addr, pgoff_t *pgoff) { *pgoff = vma->vm_pgoff + ((addr - vma->vm_start) >> PAGE_SHIFT);

return __kvm_gmem_get_policy(GMEM_I(file_inode(vma->vm_file)), *pgoff); }